Leaf Shutter - Speed testing

[DannL.]

Revisiting an old Betax No. 5 that I know has always suffered from speeds that are off. When using the record player set to 78 rpm and measuring the degrees of rotation, I calculated ~.0021 seconds per degree of rotation. Does that sound like an accurate figure?

 

[RalphLambrecht]

how do you know that the 78 rpms are accurate?Do you havemains stabilization?

 

[RalphLambrecht]

Thanks Tom1956, I think you'll understand I'm not disappointed at all... I've got shutter speeds I can use.

It's interesting how far the speeds are from nominal, which illustrates the often-heard expression: your light meter is probably the most accurate piece of equipment you have..

sometimes precision is more important than accuracy

 

[RalphLambrecht]

+1
I assembled a sensor, based on Ralph's tester, and then made a jig out of MDB to hold both a lens-on-lensboard and a 75W flood in position, carefully aligning the sensor to the lens axis. Then used free download of the Audacity program to record the waveform on my PC screen. Then the program is able to measure the time from halfway up the rising wave (shutter opening) to halfway down the falling wave (shutter closing). All six of my large format lenses in leaf shutters checked out within tolerance, except for a couple of speeds, which I then labeled on the lensboard so I could make exposure corrections when using those speeds. I did, however, see a frequency of 120 Hz superimposed on the Audacity wave (which did not affect the ability of determine the shutter speed), but I was curious to learn the cause. So I contacted Chris Woodhouse (co-author of Way Beyond Monochrome 2nd Ed., WBM2) who is the electrical guru of this checker. His response to my query:

"It is the mains - but you commonly get 2x the mains frequency (100Hz in the UK and 120 in the US). It normally comes from the fact that any power supply that has a bridge rectifier will cause current bumps 120 x per second."

I've not tried my setup with a battery-driven light source yet to see if this oscillation goes away, because I'm satisfied with the results I got.

Yes, Chris was thr electronic brain behind the shutter tester shown in WBM2nd Ed.it works wellwith audCITY AND I use it regulary.

 

[Bill Burk]

how do you know that the 78 rpms are accurate?Do you have mains stabilization?

As I understand, the 60-cycle frequency of our mains in the states are guaranteed to a very high level of precision... A friend of mine works at a power company and told me how the breakers kick out any circuit that deviates even slightly in frequency. It's the voltage that fluctuates freely around 120 nominal and I wouldn't be surprised if it varies 5 volts.

The old record player I used in the test has a synchronous motor, and its speed is governed by the mains. Now I know not to trust an old record player speed to be engineered at "exactly" 78 RPM. And it's not. I calibrated it (the old one runs at 79.2 RPM) to a German-engineered Dual CS-5000.

That particular turntable has a unique design. It's belt-driven with a computer controlled speed regulator. A small bar on the platter passes between a light source and a photo-detector on the base. Once per revolution the turntable speed is checked by a quartz-controlled clock. Any deviation from the selected speed and the needle lifts.

p.s. Shots from the camera are pleasingly well exposed, since I calibrated the shutter speeds. It is rewarding to get properly exposed negatives from a vintage camera. Now I see a need for hand-holding skills and scale focus skills, because many of the shots suffer camera shake and focus mistakes. But that's something I can work on.

I'll recommend performing this procedure (calibrating vintage shutter speeds) on particularly robust cameras and lens/shutters. For example, this is where you can appreciate the value of a high quality-build camera, because you can trust that tests like this remain valid for long enough to be worth the investment in time.

 

[Tom1956]

As I understand, the 60-cycle frequency of our mains in the states are guaranteed to a very high level of precision... A friend of mine works at a power company and told me how the breakers kick out any circuit that deviates even slightly in frequency. It's the voltage that fluctuates freely around 120 nominal and I wouldn't be surprised if it varies 5 volts.

The old record player I used in the test has a synchronous motor, and its speed is governed by the mains. Now I know not to trust an old record player speed to be engineered at "exactly" 78 RPM. And it's not. I calibrated it (the old one runs at 79.2 RPM) to a German-engineered Dual CS-5000.

That particular turntable has a unique design. It's belt-driven with a computer controlled speed regulator. A small bar on the platter passes between a light source and a photo-detector on the base. Once per revolution the turntable speed is checked by a quartz-controlled clock. Any deviation from the selected speed and the needle lifts.

p.s. Shots from the camera are pleasingly well exposed, since I calibrated the shutter speeds. It is rewarding to get properly exposed negatives from a vintage camera. Now I see a need for hand-holding skills and scale focus skills, because many of the shots suffer camera shake and focus mistakes. But that's something I can work on.

I'll recommend performing this procedure (calibrating vintage shutter speeds) on particularly robust cameras and lens/shutters. For example, this is where you can appreciate the value of a high quality-build camera, because you can trust that tests like this remain valid for long enough to be worth the investment in time.

Logically, it's the most sure and foolproof way. Burns up a little film, but still the truest way. Proof positive the camera does not lie; to quote an old expression (I think Barney Fife said that).

 

[Bill Burk]

how do you know that the 78 rpms are accurate?Do you havemains stabilization?

Ralph,

Do you have any idea how hard I was laughing while answering your question? The best humor has a kernel of truth in it.

I couldn't have made up a funnier answer than saying I calibrated to German engineering... and it's really what I did.

 

[RalphLambrecht]

German engineering is no onger what it was cracked up to be.what you need is German engineering with Jspanese quality controland ,I do know a good deal about both.

 

[yulia_s_rey]

Seeing you have many suggestions I decided to add another one to the mix. I rigged a simple one for my DIY cine cameras, simply a photodiode hooked up via a minijack to a sound card (or mic jack on Mac) and used Soundboth although audacity can work too. Lightsource -> Shutter -> Photodiode connected to PC.

 

[jerrybro]

German engineering is no onger what it was cracked up to be.what you need is German engineering with Jspanese quality controland ,I do know a good deal about both.

I think the engineering is still good, but the manufacturing, for one reason or another, is not what it used to be.